_* 
38 
gaimardiajia, II. yarvula (fig. 3), DavcUia hrrriprs (fi<;. 4). I). soUda, 
Aspleniiim cuhanicitnu A. marrophijffunK A- nnirafnm, A. Helangeri, 
Ptcris ensiformis^ Monogmmma, Anfropfnjum latlfoJ'umi, rohjpodlnm 
cucullatum and P. Phymatodes, In Iluimia gainuirdlana ami DavalVia 
hrevipes these pores reach a depth of -three-fourths to four-fiftlis of the 
thickness of the wall. In leaving these lobes of thick -walls thin, the plant 
makes the most economical mechanical use of its plastic material, for the 
lateral walls effectively reinforce the outer ones at the edges. Straight, 
lateral walls act less effectively in the same way and a few ferns have the 
outer walls thinner around the even margin, as is the case in Nrphrodium 
1712^ or they are pitted around the margin, as in Asplenium sqnarmdatum 
(fig. 5). On the other hand, Scolopendrium pinnatum, the epidermal 
cells of which are rather large in proportion to the thickness of the outer 
and lateral walls, has the outer wall reinforced centripetally for a short 
distance from each of the obscure entrant lobes (fig. 0). The same is 
true of some of the entrant angles in Athyrinm sUvaticum, whereas Asple- 
nium scandens has the outer wall thinned in the excurrent lobes and 
reinforced from the entrant curves. • ■ ■ 
The "spicular" idioblasts of the Vittariece (figs. 7, S) have long been 
familiar objects. They are wanting in Antroplupnn lafifoJ'nim and 
Vittaria minor, the former growing in sheltered places and being too 
broad to need any longitudinal reinforcement, and tlie latter being very 
small and stout. 
It has just been pointed out that wavy, lateral walls are a reinforce- 
ment of the outer wall, contributing greatly to the rigidity and strength 
of the epidermal framework as a whole. On other than mechanical 
grounds, most terrestrial ferns have the outer walls much thicker above 
than below.. In evident correlation with this fact, these ferns almost 
invariably have wavy walls for the nether epidermis alone, or have the 
latter decidedly more wavy than those above. Diplazium- furnishes many 
good illustrations. Wavy,. lateral walls, by reinforcing the outer walls 
or in extreme cases, dividing the cell into comparatively small lobes or 
parts, make larger epidermal cells possible than would otherwise be 
■tolerated from mechanical reasons. The relation of size of cell and 
waviness of wall to the stomatal movement will be discussed presently. , 
The relation of the thickness of outer wall to ' size of cell needs no 
argument; it is well illustrated within one plant by Odontosona rdusa, 
which, though a terrestrial fern along creeks, has the outer wall 1.5 /i 
thick above, and 3 /i to 4 /x thick below, the cells above being remarkably 
narrow. 
iS^foww^a,— Stomata occur on the upper surfaces of the fertile frond>^ of 
Achrostichum and Cheirophuria and on all four faces of those of Mono- 
gramma; otherwi>^e they are entirely confined to the nether surfaces of all 
